This invention pertains generally to guided missiles, and particularly to inertially stabilized platforms in such missiles.
It is common in the art to mount the antenna of a radar on an inertially stabilized platform carried by a guided missile (hereinafter sometimes referred to as a "missile") in flight. With such an arrangement the antenna may be stabilized in inertial space regardless of any maneuvering of the missile. Thus, the antenna may be mounted on a double gimbal assembly wherein torque motors are comranded to drive the double gimbal assembly to null the rates sensed by appropriately aligned gyroscopes to effect the desired stabilization.
Unfortunately, friction between parts in the double gimbal assembly and torque motor drives may prevent the degree of nulling required to maintain the position of the boresight line of the antenna in inertial space. Any such motion of the antenna in inertial space due to friction is taken by the guidance system to mean that the target has moved. As a result, then, guidance command signals are generated to cause unwanted maneuvering of the missile.
The problem is exacerbated when the size and inertia of the components of the guidance system are reduced. The torque available to achieve nulling is also reduced because the torque motors are necessarily smaller so that additional gearing may be required between each torque motor and the associated one of the gimbals in the dual gimbal assembly to provide sufficient torque. It will be appreciated that friction in the additional gearing may adversely affect performance. In particular, as the gimbal rate decreases toward zero from either direction, a point is reached where the friction increases to such a degree that insufficient torque is applied to effect a desired movement. The result is that an error in the sensed line of sight rate appears, making it seem to the guidance system that the target has maneuvered even though no maneuver has actually occurred. Guidance command signals then are generated to counteract a nonexistent maneuver of the target, thus causing the missile to be incorrectly guided. In the terminal phase of flight of a missile any error in guidance is to be avoided because it may not be possible to complete corrective maneuvers before intercept.